In imaging and infrared thermography, it is conventional to use so-called “uncooled” detectors operating close to the ambient temperature, this term for example applying to temperatures ranging between −40° C. and +90° C. Such detectors may be temperature-regulated at their focal plane, typically by means of a Peltier cooler (“TEC”). More generally, the focal plane is however free of having a variable temperature (“TEC-less”). Such devices use the variation of a physical quantity of an appropriate material, according to temperature, around 300 K. In the most current case of bolometric detectors, this physical quantity is the electric resistivity.
An elementary detector of such a detection device generally associates:                means for absorbing the infrared radiation and for converting it into heat;        means for thermal isolating the detector, to enable it to heat up under the action of infrared radiation;        thermometry means which, in the context of a bolometric detector, use a resistive element;        and means for reading the electric signals provided by the thermometry means.        
This type of detectors intended for infrared imaging is conventionally made in the form of a bidimensional array of elementary detectors, or bolometers, each elementary detector of said array being formed of a membrane suspended via supporting arms above a batch-manufactured support substrate. Such an elementary detector array is usually called an imaging “retina”.
Electronic means for sequentially addressing elementary detectors and for forming an electric signal in relation with each bolometer, and for then possibly processing said signal in more or less sophisticated fashion, are further provided in the circuit, usually made of silicon. The general electronic system directly connected to the bolometers is known as “read out integrated circuit” (ROIC).
To obtain the image of a scene via this detector, the image of the scene is projected through an adapted optical device onto the elementary detector array, this array being placed in the focal plane of the optical device. Rated electric stimuli are applied via the read circuit to each of the to elementary detectors, or to each row of such detectors, to obtain an electric signal forming the image of the temperature reached by each of said elementary detectors. This electric signal directly depending on the electric resistance of each elementary detector is used by the application system integrating the detector, to form the thermal image of the observed scene. Such a system is conventionally called “camera”.
Now, it is generally observed that bolometric materials currently used to manufacture an imaging bolometer, such as for example amorphous silicon (a-Si), silicon germanium alloy (a-SixGe1-x), or vanadium oxide (VOx), have an electric resistance which more or less drifts along time, for exposure conditions corresponding to the normal use of the detector.
More critically still, a drift can also be observed when the detectors have been momentaneously irradiated by a very intense infrared source, for example, the sun or strong radiative sources (projectors, etc.). In this case, term “remanence” is preferably used since the “drift” does not concern all the elementary points of the detector, but only the areas corresponding to the image of the hot source through the optical device, which form a remanent “image” after the source having created this mark has disappeared. In this case, it is often spoken of a “ghost image” (or “sunburn”), which disappears more or less rapidly along time, since the sensitive material of the image points concerned by this high irradiation has been temporarily modified, and then returns at a variable speed to its state before irradiation. Such artifacts adversely affect the image quality, and most of all locally distort the detector calibration, that is, the ratio of the output signal to the temperature of the observed scene.
In the context of the present invention, general term “drift” of a bolometer characterizes the fact that, for given environmental and operating conditions, called “reference conditions” hereafter, such as for example the incident radiation on the bolometer, the ambient temperature for it (internal camera temperature), and the electric read signals, the electric characteristics of the bolometer have drifted away along time from their initial reference values, which can be observed in such conditions, especially in a specific exfactory acquisition operation, called calibration, before the putting into service of the detector.
In the context of the present invention, the difference observed between the signal obtained during the calibration operation and the signal which would be obtained at any subsequent time in the detector lifetime if it was placed back in the exact same conditions, is called “drift”, be it a variation resulting from a slow evolution, generally spatially uniform, of the sensitivity characteristics of all the detector bolometers, in relation with the relative natural instability of the currently-used thermometric materials, or much faster variations (on appearing thereof, and as they subsequently relax) spatially distributed and variable along time, resulting from too long an observation of an intense radiation source.
To provide a proper introduction to the following, the most current calibration process of the state of the art will be detailed.
Very generally, this calibration process comprises the elaboration of bidimensional correction parameters (usually called “tables”) of offsets and gains of the array retina, which are then used during the detector operation to correct the bolometer characteristic dispersions.
The offset table is obtained by measuring and storing all the output signals obtained when facing a first scene of uniform temperature (for example, a reference black body taken to a first temperature T1). The output signal (or continuous level), is called NCT1, and the specific signal of a bolometer of coordinates (i,j) in the bolometer array is called NCT1(i,j). The offset table thus simply gathers all values NCT1(i,j), which represent the distribution (the dispersion) of the output signal in such reference conditions.
The detector is then placed in front of a different second scene of uniform temperature, for example, a second reference black body taken to a second temperature T2, and a new table NCT2(i,j) is acquired and stored.
Reference gain table Gref(i,j) defined by relation:
            G      ref        ⁡          (              i        ,        j            )        =                              NC                      T            ⁢                                                  ⁢            2                          _            -                        NC                      T            ⁢                                                  ⁢            1                          _                                      NC                      T            ⁢                                                  ⁢            2                          ⁡                  (                      i            ,            j                    )                    -                        NC                      T            ⁢                                                  ⁢            1                          ⁡                  (                      i            ,            j                    )                    is then calculated from tables NCT1(i,j), NCT2(i,j), and respective algebraic averages NCT1 and NCT2 of these tables. In the rest of the present document, a V type notation, where V is a “table” of scalar values of same dimensions as the retina, represents the algebraic average of this table.
Table Gref(i,j) shows the relative distribution (dispersion) of the response or responsiveness of all the retina bolometers.
Such offset and gain tables, resulting from the calibration, are stored in the camera and used, after its putting into service, by a calculation unit integrated to the camera to perform the to conventional “two-point correction” of each raw signal S(i,j) of each image frame, to obtain corrected signal Scorr(i,j) according to the following relation:Scorr(i,j)=Gref(i,j)·(S(i,j)−NCT1(i,j))+NCT1
Each raw signal S(i,j) thus has its individual difference with respect to the average of the imager corrected in terms of offset and gain.
Usually, the camera is provided with a shutter, interposed between the optics and the sensitive focal plane, intended to form the equivalent of a uniform thermal scene at the shutter temperature.
When it is considered that output signal Scorr(i,j) is no longer sufficiently accurate, for example, due to a temperature drift or to the temperature dispersion of the focal plane, especially for “TEC-less” detectors having no temperature regulation, or due to the small individual or general drifts of bolometers, the shutter is activated and offset table NCT1 is updated, for example replaced, with a new table NCshut of the raw output signals corresponding to the closed shutter.
The conventional use of a shutter thus provides a simple and accurate way to form satisfactory offset tables. However, the observation of a scene which is naturally essentially uniform at ambient temperature, with no shutter, or the use of means for uniformizing the scene radiation, for example, by defocusing of the image, is also possible in the context of the present invention. Hereafter, to simplify notations, it will be written that table NCshut “corresponds to the shutter” without for this to necessarily imply the presence of a mechanical shutter.
The images obtained after reopening of the shutter are then efficiently corrected again by means of the new offset table.
It is usually considered that it is not necessary to update the gain table, since the physical phenomena causing the relative response dispersions, as compared with the average value, practically do not vary during the entire detector lifetime.
Indeed, response Resp of a voltage-biased bolometer (that is, submitted to a constant voltage and with no other external constraint on the current flowing through the bolometer) can generically be expressed as follows:
                              Re          ⁢                                          ⁢          sp                =                                            ∂              S                                      ∂                              θ                scène                                              ∝                                    1                              R                                  a                  ⁢                                                                          ⁢                  c                                                      ⁢            A            ×            ɛ            ×            TCR            ×                          R              th                        ×                                          ∂                                  Φ                  ⁡                                      (                                          θ                      scène                                        )                                                                              ∂                                  θ                  scène                                                                                        (        1        )            where:                Rac is the electric resistance of the bolometer;        A is the area of the bolometer dedicated to the absorption of the radiation;        Rth is the thermal resistance between the bolometer membrane and the substrate above which it is suspended;        ε is the effective optical coupling (or absorption) coefficient of the membrane;        TCR is the variation coefficient of the electric resistance of the bolometer according to its temperature;        Φ is the incident radiative energy flow on the bolometer; and        θscène is the scene temperature.        
The response is thus partly determined by architectural or design parameters, such as area A, thermal resistance Rth, coefficient ε, and various parameters of the optical system equipping the detector appearing in last term Φ(θscène).
Now, it can be observed that the value of this first group of parameters, to which the parameters of the read circuit intended for the forming of raw signal S(i,j) can be added, does not substantially vary along time and thus remains substantially constant all along the detector lifetime.
Another group of parameters is however involved in the bolometer response, in the form of the characters associated with the thermometric material thereof, that is, its electric resistance Rac (through its resistivity ρ) and its coefficient of relative variation of the resistance according to temperature TCR defined by the following relation:
                    TCR        =                              1                          R                              a                ⁢                                                                  ⁢                c                                              ·                                    ∂                              R                                  a                  ⁢                                                                          ⁢                  c                                                                    ∂              T                                                          (        2        )            where T is the temperature of the bolometer membrane.
These material parameters are capable of significantly drifting, as already noted. In particular, electric resistance Rac of usual bolometry materials (essentially vanadium oxide and amorphous silicon) may intrinsically vary by a few percents along the detector lifetime. This resistance may also vary under the effect of a particularly high heating caused by an intense and/or prolonged infrared irradiation, which is more prejudicial in terms of image quality due to the fact that the to disturbance generally only concerns part of the retina. An “artificial” contrast spot which is not related to the scene thus forms on the image, said spot disappearing after correction of the sole offset, that is, after updating the “one-point” correction, but where the thermographic calibration is distorted since the initial gain table is no longer accurate on the retina portion which has been modified.
Such calibration variations or drifts may be significant as compared with the accuracy of the images or thermal measurements which are intended to be obtained. Thus, the gain table used in the “two point” correction requires being regularly recalibrated if a constant image quality is desired during the detector lifetime.
Methods for correcting these drifts have thus been provided, for example, in document FR 2936052. This document teaches, for a detector equipped with a shutter and with a Peltier cooler regulating the temperature of the focal plane with respect to a reference temperature, the correction of a responsiveness drift due to time or spatial variations of bolometric resistances. The implemented principle is based on a device comprising a reference resistance, itself subject to the considered drift, and means for measuring the drift of this reference resistance with respect to its initial value. These means are activated each time an update of the gain table is deemed necessary due to a drift of the bolometer responsiveness. The correction comprises, while the detector is “in service”, acquiring the raw signals corresponding to the shutter, deducing from these signals a relative variation of this reference resistance with respect to its initial value, and then multiplying the gains of the gain table by a factor proportional to said relative variation.
Detector “in service” means that the correction may be performed while the camera is for example available for the user, by a procedure which does not use the calibration methods applied by the constructor in factory, especially by means of black bodies.
The performed correction may be general (unique and valid for the entire retina) or individual (adapted to each sensitive point) in the case where the reference resistance is the bolometer itself. The gain table thus updated is then applied to the correction, for example, digital, of the raw signals. A substantial correction of errors due to the responsiveness drift is thus obtained.
This method is efficient in that it suppresses most of the calibration differences, and, in case of an individual implementation, also most of the drifts due to intense local irradiations.
It can however be observed that “remanent”-type spatial responsiveness disturbances due to an to intense local irradiation are not perfectly corrected in all operating conditions of the camera. Indeed, when the temperature of the shutter during the calibration is close to the average temperature of the scene observed afterwards, the “two-point” corrected image resulting from the updated offset and gain tables is satisfactory.
However, when the camera temperature, and thus the shutter temperature, is remote from the average temperature of the scene, there remains after the “two-point” correction a well apparent ghost image of the previously-disturbed areas, in particular on images of low thermal contrast, which tend to be overcorrected, thus resulting in an inversion of the local contrast on the image after correction.
FIG. 1 is a thermal image obtained from a bolometric detector of the state of the art based on amorphous silicon implementing the teachings of document FR 2936052 and illustrating the overcorrection phenomenon. The image of FIG. 1 is obtained after a “two-point” correction of the state of the art, after the offset table has been corrected while the camera, and thus the shutter, had an average temperature of 60° C. The observed scene has an average temperature of 30° C. The grey scale of this image assigns a light shade to higher temperatures, the general dynamics of the image in terms of scene temperature being on the order of 4° C.
The dark spots are the “ghost images” before the two-point correction due to the presence of the sun in the image field of the detector in a use prior to the acquisition of the image of FIG. 1, according to various exposure times, at several points of the retina. Such spots appear in very light contrast when there is no correction, but appear in dark contrast on the corrected image, which means that these spots are overcorrected.
This example of the imaging field is also an illustration of prior art limitations in thermography. Indeed, the scene temperature estimate which would be extracted from the points located in the disturbed areas would be distorted with respect to the correct measurement provided by the neighboring undisturbed points.
Such imaging and calibration defects are correctable by means of a factory recalibration of the detector, by repeating the process of exposure to two black bodies at two reference temperatures T1 and T2, to determine a new gain table, as previously described. However, in addition to the fact that such a complex operation is impossible to implement for systems in service, that the camera should be replaced in the same thermal conditions as during the calibration, it should be repetitively reproduced in relaxation phases after an excessive irradiation. In fact, this solution cannot be envisaged without putting the camera out of service for a long time.
Document U.S. Pat. No. 7,030,378 provides a “field” approach based on two compared measurements of the electric resistances of the bolometers, obtained at two different operating temperatures of the camera itself, after initial factory calibration. In addition to the fact that this method requires a significant variation of this temperature to provide an exploitable result, it is not capable of providing a solution to the problem of insufficient calibration accuracy when the shutter is taken to a temperature very different from that of the scene.
There thus remains an obvious need to have a method capable of substantially removing residual calibration errors, and more specifically detrimental artifacts in imaging and thermography consecutive to periods of excessive exposure to an intense thermal radiation, always observed after a correction according to the state of the art.
There further remains a need to have a protocol for correcting drifts and said artifacts which does not require using specific conditions requiring putting the detector out of service for too much time.
There also remains a need to provide a protocol for correcting drifts or said artifacts applicable to cameras in service provided with standard state-of-the-art read circuits and further valid whatever the thermal form of the detector, that is, having a regulated focal plane temperature or of non-regulated type (“TEC-less”).